Gene transferring vectors are used in research and gene therapy to express foreign genes in target cells. In such situations, it is sometimes desirable that two genes be expressed in the same target cell. This will allow, for example the selective proliferation or death of target cells to which therapeutic genes have been inserted by expressing therapeutic genes in combination with selective genes. Alternatively, this will allow the monitoring of the dynamics of a therapeutic transgenic cell in vivo by expressing marker genes (e.g., GFP etc.) in combination with therapeutic genes. Furthermore, this will allow the expression of proteins that function by forming a complex between two types of subunits, such as receptors and transcription factors.
Previously, as vector systems for coexpression of two genes, a form in which multiple promoters are inserted, and another form in which one promoter is combined with an IRES (Internal Ribosomal Entry Site) sequence have been reported. However, the expression properties of these vectors are by no means satisfactory.
For example, vectors having multiple promoters suffer from the problem of efficient expression from only one of the promoters due to interference among the promoters. Alternatively, vectors with a combination of one promoter and an IRES sequence contain the problem that the expression level of genes on the 3′ side from IRES is only ⅕ to 1/10 of that on the 5′ side from IRES.